This invention relates to a transparent crystallized glass for use in a front window or an inspection window of a combustion apparatus for burning oil, coal, gas, wood, or the like, i.e., a room heater, a heating furnace, an annealing furnace, and so on.
A front window of a room heater or the like serves to enhance a room-heating effect by transmitting heat radiation emitted from a flame inside the heater to the outside and to visually increase warm feeling by allowing the flame to be visible. An inspection window serves to allow a burning condition of the flame to be observed from the outside. These windows must be resistant against a high temperature produced from the flame and heat shock at the ignition. Therefore, a material used in the windows of such combustion apparatus is required to be transparent and to be low in thermal expansion and high in mechanical strength so that the heat resistance and the heat-shock resistance are excellent.
At present, use is made of a borosilicate glass, a silica glass, and an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass in the windows of the room heater and the like. However, the borosilicate glass is not sufficient in heat resistance and heat-shock resistance. The silica glass is excellent In thermal characteristics but is expensive. On the other hand, the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass is excellent in heat resistance and heat-shock resistance because the coefficient of thermal expansion is small and the mechanical strength is high, and can be produced at a relatively low cost. Therefore, this glass is widely used.
However, the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass is disadvantageous in that, when It is placed in a combustion atmosphere, an inner surface or a burning-side surface of the window is subjected to chemical corrosion to produce microcracks which result in considerable decrease in transparency and mechanical strength.
The reason why the above-mentioned disadvantage is caused is as follows. In the combustion atmosphere of the combustion apparatus for burning oil, coal, gas, wood, or the like, SOx produced from sulfur contained in such fuel is present and reacts with H2O produced by combustion to produce H2SO4. H+ions produced from H2SO4 cause an ion exchange reaction with Li+ ions in the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass to shrink the volume of crystals, resulting in occurrence of the cracks.
In order to avoid the above-mentioned disadvantage, use is made of a technique of forming a coating film such as SiO2 on the surface of the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass. However, this technique is unfavorable because the production cost is increased. Alternatively, it is possible to suppress the occurrence of the ion exchange reaction with the H+ ions by reducing the content of the Li+ ions in the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass. However, there arises a problem that the transparency is deteriorated and the coefficient of thermal expansion is increased.
It is therefore an object of this invention to provide an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass which will not produce microcracks even if it is exposed to a H2SO4containing atmosphere produced by a fuel such as oil, coal, gas, wood, or the like for a long time.
As a result of various experiments, the present inventors have found that, by reducing the Li+ concentration on the surface of a crystallized glass as compared with the inside, the occurrence of microcracks resulting from an ion exchange reaction with H+ ions can effectively be suppressed, and herein propose this invention.
Specifically, an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass according to this invention comprises Li2O, Al2O3, and SiO2 with xcex2-quartz solid solution precipitated as main crystals and is characterized in that the Li+ concentration ratio at a position of 50 nm from the surface of the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass is equal to 0.80 or less where the Li+ concentration at a position of 2000 nm from the surface is supposed to be equal to 1.